Open your inventory, hover your cursor over the mon you want to relearn a move to and hit the "Moves" button on the drop-down menu.

On the very right of your screen you'll see a trash can, to delete a move of choice first select one, then single-click said bin: your move is now gone for good.

Now go on and delete all 4 moves, this will force new moves to be generated, these are randomly picked from the movepool your Pokémon has access to at the level it is at.

Here's the tricky part: the moves you see now are just a visual thing and will be lost after a relog, but do not worry, just engage in battle with a wild mon; the moves you see now and can select from are the ones you'll end up with.

Not satisfied with the moves? Just repeat the deleting & refreshing process again until you do get the moves you want.

~Note: If you're aiming for a specific move, use this method ASAP, as doing it on a higher level just means there are more moves the system will choose from.

To reiterate what I said previously, by using this glitch you will lose the Pixelmon's current moves, therefore, you will not be able to have the combination of Egg Moves/Pre-Evo Moves + early level Moves in a Moveset.

Just to name a few examples I'll use the mons Scizor & Donphan again:

It is illegal to have Baton Pass and Bullet Punch on a Scizor, as the former is an Egg Move and BPunch requires the use of this glitch.

It is illegal to have Odor Sleuth and Rapid Spin on a Donphan, as the former is obtained by Pre-Evo and RSpin requires the use of this glitch.